Wind turbines are an established means for generating electricity. A wind turbine generally comprises a rotor supporting a plurality of blades. The rotor is arranged to drive a generator directly or via a gear box. In a horizontal axis turbine, the rotor and the housing of the generator (the housing is also called a nacelle) are supported by a tower. The up-wind type where the blades face into the wind is most common. A yaw system keeps the orientation of the rotor and the blades within allowable limits relative to the wind direction. The turbine control system is controlling the rotor speed and power by pitching the blades. Yaw and pitch motions are commonly powered by geared electric motors. Hydraulic cylinders are also commonly used for powering pitch motion. Ultrasonic or mechanical instruments for measuring wind speed and direction are placed on top of the nacelle. For large wind turbines, the wind flow may not be uniform across all of the rotor-swept area and the ability to pitch the blades individually is therefore common.
The rotor is supported by bearings, which are usually rolling element bearings and are sometimes also referred to as low friction bearings. For large wind turbines, there is a trend towards using large-diameter slim-profile bearings. The large-diameter slim-profile bearings are arranged coaxially and are spaced closely together. The bearings may also be combined in one single bearing unit. The result is a compact design with a high bending- and torsional stiffness. The use of large-diameter slim-profile bearings puts more complex requirements on the design and stiffness of the adjoining components, when compared to conventional, smaller wind turbine designs.
The rolling elements are preferably kept under sufficient contact forces to ensure rolling contact with the bearing races. Loss of contact forces could cause the rollers to start skidding against the bearing races, which would be detrimental. Maintaining contact forces is achieved through imposing a high pre-loading of the bearings during the assembly and installation process of the bearings. The pre-loading ensures a minimum initial contact force between the stationary part of the bearing, the rolling elements and the rotary part of the bearing. The pre-loading also avoids excess localised contact forces within the bearing after the rotor and blades are attached to the bearing. The contact forces between rollers and races resulting from the combination of the bearing pre-loading and the external forces from operation present design constraints of the bearing. Flexing of the slim-profile large-diameter bearings under operation can cause an uneven load profile around the diameter of the bearing. Furthermore, wear of the rollers and races will reduce the initial pre-loading over time. Global and spatial variations in contact forces will be measurable as variations in strain in the bearing races.